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Sounds like the catalytic converter or muffler maybe getting plugged up and the computer is putting the truck in limp home mode. Did you put a scanner on the truck. You might try to remove the 02 sensor ahead of the cat and see if it runs a little better it will not be perfect but should stay running if so do an exhaust check. When the cat or muffler gets plugged it puts back pressure on the engine and won't allow the fuel to be pulled into the engine properly.

Either Park or Neutral is fine, but first you need to warm up the truck fully and then shift into every shifter position, pause for several seconds in each position, then LAST shift into either Park or Neutral to check the fluid level. The engine must remain running.

You need to put a scanner on the truck and see if any codes are set and if so of course repair them and then clear the codes in the computer with the scanner. I would check the idle air control and the throttle position sensor. I would also check the alternator by starting the truck and then taking off the positive battery cable and the truck should continue to run. If not the alternator that was put on is bad.

there would have been enough induced voltage flying back through the electrical system to keep the motor running.This is not always a good test.A little bit more explanation would be helpful.Does that check gauge remain on or off while driving normally?When it goes off or on does it do it spasmodically or does it do it until you restart the engine?Regards

* Built-up carbon in the ignition chamber can glow red after the
engine is off, providing a mechanism for sparking unburnt fuel. Such a
thing can happen when the engine runs very rich, depositing unspent fuel
and particles on the pistons and valves. Similarly, non-smooth metal
regions within the piston chamber can cause this same problem, since
they can glow red. It has also been suggested that an improperly rated
sparkplug can retain heat and cause the same problem.
* A carburetor that does not close entirely can contribute to
running once the engine is off, since the extra fuel and oxygen mixture
can combust easily in the warm piston chamber. Similarly, hot vaporized
oil gases from the engine crankcase can provide ample fuel for
dieseling.
* Incorrect timing.
* An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.
* An idle speed that is too fast can leave the engine with too much
angular momentum upon shutdown, raising the chances that the engine can
turnover and combust more fuel and lock itself into a cycle of
continuous running.

Almost sounds like the timing is too far advanced and you experiencing
engine run on. Have you done any work around the distributor lately? Or
had the timing set?

If it was the switch itself I'd think it would either shut off right
away or not at all unless the key was giggled to the point where it
broke all power to the ignition system.

Other possible causes could be to high of an idle speed, worn timing chain or bad O2 sensor.Keep us updated.

Idle speed is not related to the clutch position. With the pedal depressed, the engine is freewheeling and is not engaged with the remainder of the driveline. Likely the idle speed is the same with the engine running in neutral or with the clutch depressed. check the idle air control and clean the throttle plate in the throttle body as well as checking the throttle cable for binding etc. If there is a small vacuum leak, that also can raise idle speed.

I faulty Idle Air Control valve (IAC) can cause these specific symptoms. You could try diconnecting the IAC valve (usually near the throttle body) and regulating the idle yourself. If the fluctuations go away and you are able to hold a constant idle position with the gas pedal (if vehicle will start), the IAC is the problem.